X-ray tube comprising a liquid-cooled anode

ABSTRACT

The cooling side of the anode target plate of an X-ray tube is provided with a surface-increasing cooling structure. An injection device for a cooling liquid is mounted against the cooling structure such that the cooling liquid is forced to flow through ducts present in the cooling structure. At least the material of the surface of the cooling structure is preferably silver.

United States Patent Diemer et al.

[4 1 Oct. 21, 1975 [56] References Cited UNITED STATES PATENTS 2,886,7235/l959 Burley 313/32 Primary ExuminerRud0lph V. Rolinec AssistantExaminerDarwin R. Hostetter Attorney, Agent, or FirmFrank R. Trifari;Drumheller Ronald L.

[57] ABSTRACT The cooling side of the anode target plate of an X-raytube is provided with a surface-increasing cooling structure. Aninjection device for'a cooling liquid is mounted against the coolingstructure such that the cooling liquid is forced to flow through ductspresent in the cooling structure. At least the material of the surfaceof the cooling structure is preferably silver.

6 Claims, 3 Drawing Figures US. Patent Oct. 21, 1975 SheetlofZ 3,914,633

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US. Patent Oct.21,1975 Sheet2of2 3,914,633

\ y NAM X-RAY TUBE COMPRISING A LIQUID-COOLED ANODE The inventionrelates to an X-ray tube provided with an anode comprising an anodetarget plate which comprises, arranged opposite to each other, a targetfor an electron beam to be directed thereon and a cooling surface forgiving off heat to a flowing cooling medium.

In known X-ray tubes of this kind it was found, that in spite of thecooling, the anode target plate becomes hot such that it is damaged andthe service life of the tube is reduced. One of the causes thereof isknown to be the presence for some time of gas bubbles on the coolingsurface due to an insufficiently turbulent liquid flow. On the basisthereof, various improvements have been proposed which aim to increasethe turbulence in the liquid flow.

For example, US. Pat. No. 2,886,723 describes an X-ray tube in which alongitudinally injected liquid flow is forced to a higher degree ofturbulence by projections on the boundary walls of the flow duct.Netherlands Pat. No. 77,920 describes an X-ray tube in which, for thesame reasons, a rotating disc is arranged in a transverse injectedliquid flow near the cooling surface. Netherlands Pat. No. 74,278describes an X-ray tube incorporating a transverse multiduct injectiondevice for the cooling medium. In each of these known embodimentsimproved heat transfer between the cooling medium and the coolingsurface is indeed realized. As a result, the anode target plate becomesless hot and the service life is prolonged.

Particularly in the case of X-ray tubes having a comparatively smalltarget spot area, i.e., the cross-section of the electron beam at thearea of the target, however, the anode target plate is stillcomparatively quickly damaged. It was found that this damage consistsmainly in the local roughing of the target in and near the target spot.In addition to a reduced service life of the tube, this also causes acontinuous reduction of the radiation efficiency of the tube.

The invention has for its object to provide an X-ray tube in which thetarget plate is substantially less readily damaged, also in the case ofcomparatively high local loading. To this end, an X-ray tube of the kindset forth according to the invention is characterized in that the anodetarget plate is comparatively thin, measured between the target and thecooling surface, and is provided with surface-increasing recesses at thearea of the cooling surface, the flow path for the cooling medium beinglimited at least mainly to these recesses at these areas.

As a result of the provision of the surface-increasing structure in theanode target plate, an X-ray tube is obtained having a substantiallysmaller reduction in radiation efficiency and a longer service life. Acontribution in this respect is made by the improved thermal contactbetween the anode target plate and the cooling medium, the largercooling surface, the higher flow rate of the cooling medium at the areaof the cooling surface as well as by the shorter heat-leakage path. Asa'result of the improved heat transfer near the cooling surface, theanode target plate may be thinner, so that it becomes less hot again.Because the anode target plate becomes less hot at the area of thetarget, the temperature gradients occurring at this area cause lessroughening of the surface. In a preferred embodiment according to theinvention, the recesses consist of adjoining isosceles pyramids whichextend approximately halfway the thickness of the anode target plate. Aflat boundary of an injection tube for the cooling medium which isdirected towards the cooling surface is mounted against the peaks of theremaining raised portions, which are again isosceles pyramids.

Because the anode target plate also becomes less hot on the coolingsurface side in an X-ray tube according to the invention, less corrosionoccurs also at this side. Any corrosion occurring at this area, however,is additionally detrimental because it attacks the cooling structure. Ifsuch corrosion occurs, the improved cooling will be lost after some timeand the thin anode target plate will quickly become completely unusable.So as to prevent this, in a preferred embodiment according to theinvention the cooling side of the anode target 7 plate is provided witha corrosion-resistant material. To this end in a closed cooling systemuse can alternatively be made of a liquid having a comparativelyslight-corrosion effect on the material of the cooling surface.

A few preferred embodiments according to the invention will be describedin detail hereinafter with reference to the drawing.

FIG. 1 is a diagrammatic representation of a preferred embodiment of anX-ray tube according to the invention.

FIG. 2 is a diagrammatic representation of a part of the X-ray tubeshown in FIG. 1 which comprises the anode target,

FIG. 3 is a diagrammatic representation of a preferred embodiment of ananode construction according to the invention.

The X-ray tube shown in FIG. 1 comprises an envelope, consisting of aglass portion 1 and a metal portion 2 which are vacuumtight connected toeach other by means of the connection ring 3. The metal portion 2comprises windows such as 4 and 5 which are vacuumtight contained insupport rings 6 and 7. The portion 2 furthermore comprises a cap 8, ananode 9 with an anode target plate 10 forming part thereof. Mounted inthe anode 9 is a cooling sleeve 11 with an inlet line 12 and an outletline 13. Provided in the cooling sleeve 11 is an opening 14 fordirecting a cooling liquid transverse to the anode target plate 10. Asealing plate 38 is provided with a guide sleeve 39 which projects intothe cooling space. The cooling sleeve 11 is preferably mounted 15 cminto the anode sleeve 9 with insertion of one or more O-rings.

Arranged opposite to the anode target plate 10 is a cathode body 16 inwhich an electron source is mounted, in this case a filament 17. Theglass envelope 1 comprises a passage element 18 with passages 19 forconnections 20 of current or voltage sources not shown.

Provided in the anode target plate 10 is a cooling structure 21 which isshown at an increased scale in FIG. 2. In addition to the anode 9, thecooling sleeve 11 and the filament 17, FIG. 2 shows an electron beam 22and an X-ray beam 23. In this preferred embodiment the cooling structure21 consists of isosceles pyramids 24 which are impressed in the anodetarget plate. The raised portions 25 also constitute isosceles pyramids.The target plate including the pyramids, has a thickness of, forexample, 2 mm and the pyramids have a depth of 1 mm. An end face 26 ofthe cooling sleeve 11 engages the peaks of the raised portions. In thispreferred embodiment, the pressure of the cooling liquid ensures thatthis engagement is maintained during operation. Instead of thisself-adjusting construction, the cooling sleeve can alternatively bemounted against the anode target plate under spring pressure, or canform one assembly therewith. In the latter case the cooling structureconsists of a duct system which is arranged between a cooling part and atarget plate part. The duct system should permit lateral passage of andbe in open communication with an inlet opening for the cooling medium. Acooling liquid which is pressed through the opening 14 is thus forced toflow between the raised portions. As a result, proper thermal contactbetween the cooling liquid and the target plate is ensured. If isoscelespyramids are used in the cooling structure, the cooling area of thecooling surface is increased exactly by a factor 2, the transversedimension of the target being the same. In a first approximation, thisresults in a twice as large heat transfer to the cooling liquid. Becausethe cooling liquid is forced through the more or less zigzag-extendingducts between the pyramids, any gas bubbles appearing therebetween willbe quickly taken along. As a result of the higher flow rate due to thenarrow passage opening, the cooling liquid will become less hot and theheat transfer will be increased. The raised portions, and hence also therecesses, of the cooling structure can alternatively have a differentshape, for example, the shape of half spheres, cubes, cylinders, conesetc. However, the structure should always permit lateral passage asotherwise the cooling sleeve cannot be mounted against the structure andthe flow of cooling liquid will then be restricted mainly to the spaceto be left for this purpose. In a further preferred embodiment, thecooling structure consists of a system of preferably zigzag-extendingducts which are provided in the anode target plate, for example, byetching.

As was already stated, a minor corrosion of the cooling surface quicklyhas serious consequences in comparison with known X-ray tubes. On theone hand, the cooling medium must flow through narrow so readilyclogging ducts, while on the other hand the peaks of the raised portionscan disappear after some time due to corrosion, with the result that theliquid passage can then concentrateon a resultant free passage opening.In both cases the proper, uniform cooling is lost. So as to preventthese phenomena, a preferred embodiment of an X-ray tube according tothe invention incorporates a known closed cooling system. In this systemthe heat taken up from the anode is given off in a heat exchanger. Thechoice of the cooling medium in such a system is free to a high degree.For example, a binary mixture can be used, such as water with alcohol,one component of which is subjected to an alternating phase transitionduring the cooling process.

In a further preferred embodiment according to the invention, the attackof the cooling structure is reduced by a suitable choice of thematerials of the anode target plate at the area of the cooling surface.In addition to copper, silver is suitable material for this purpose inview of its favourable heat-conductivity and high corrosion-resistance.The cooling structure can be provided with a silver layer, for exampleby vapour-deposition or in a galvanic manner.

In a further preferred embodiment according to the invention, the anodetarget plate comprises, as is shown in FIG. 2, a comparatively thintarget disc 30 and a cooling disc 31 which also serves as a support forthe target disc. The cooling disc is made, for example, of silver orcopper, whilst the target disc is made of one of the metals known to beused for this purpose, for example, copper molybdenum, tungsten, cobaltand the like. The target disc can be provided on the cooling disc bydiffusion, but any other method is also feasible, provided that thenecessary proper thermal contact between the two discs is realized.

The mutual orientation of a cooling disc 33, a cooling sleeve opening 34and a line-like target spot 35 of a further preferred embodiment areshown in FIG. 3. Linefocus tubes of this kind are frequently used fordiffraction examinations. The line-like target spot or the line focushas a width of, for example, 0.4 mm and a length of 8 mm. The coolingdisc is now mounted such that the line focus encloses an angle ofapproximately 45 with straight lines 36 along which the pyramids arearranged. The cooling sleeve opening 34 is arranged directly opposite tothe line focus, with the result that the cooling medium is injectedagainst the line focus on the cooling side. In a preferred embodiment,the cooling disc is provided with areas 37 in which no cooling structureis present. These smooth areas are arranged in the longitudinaldirection of the line focus, but are situated at least a few times thewidth of the line focus outside the line focus. As a result of thesmooth areas 37, the flow direction of the cooling medium is forced moretransverse to the longitudinal direction of the line focus.

An X-ray tube according to the invention is furthermore particularlysuitable for use in an X-ray fluorescopy apparatus which is equippedwith a so-termed end-window tube. In such tubes the target plate isarranged at a small distance from an end face of the envelope. So as toprevent damage by dispersed electrons, the anode is positive withrespect to the surroundings. Consequently, the anode target plate mustbe cooled with de-ionized water. This would cause additionally fastcorrosion of the cooling surface. In these tubes usually no space isavailable for a complex cooling system at this area. The use of an X-raytube comprising an anode target plate provided with a cooling structureaccording to the invention offers a favourable solution in such a case.

What is claimed is:

1. In an X-ray tube, an anode comprising:

a wall;

an anode target plate of heat conductive material having on one sidethereof a target area for an electron beam and having the opposite sidethereof facing said wall with an array of heat conductive projectionssubstantially increasing the heat radiation surface thereof andextending from said plate to said wall effectively forming aninterconnected system of ducts around said projections; and

means for directing cooling medium between said wall and plate forcingsaid medium turbulently through said system of ducts around said heatconductive projections, thereby cooling said target area.

2. An anode as defined in claim 1 wherein said array of projections is aregular pattern of adjoining pyramids.

3. An anode as defined in claim 1 wherein said means for directingcooling medium directs said cooling medium through said wall toward saidanode target plate.

opposing edges thereof to force said cooling medium substantially inopposite directions lateral with respect to said elongate aperture, saidprojections being positioned in a staggered array with respect to saidopposite directions to cause said cooling medium to flow in a zig zagfashion around said staggered projections.

1. In an X-ray tube, an anode comprising: a wall; an anode target plateof heat conductive material having on one side thereof a target area foran electron beam and having the opposite side thereof facing said wallwith an array of heat conductive projections substantially increasingthe heat radiation surface thereof and extending from said plate to saidwall effectively forming an interconnected system of ducts around saidprojections; and means for directing cooling medium between said walland plate forcing said medium turbulentyly through said system of ductsaround said heat conductive projections, thereby cooling said targetarea.
 2. An anode as defined in claim 1 wherein said array ofprojections is a regular pattern of adjoining pyramids.
 3. An anode asdefined in claim 1 wherein said means for directing cooling mediumdirects said cooling medium through said wall toward said anode targetplate.
 4. An anode as defined in claim 3 wherein said means fordirecting cooling medium includes an aperture in said wall.
 5. An anodeas defined in claim 4 wherein said aperture corresponds in shape andposition to the target spot.
 6. An anode as defined in claim 4 whereinsaid aperture is elongate and said wall and plate are joined alongopposing edges thereof to force said cooling medium substantially inopposite directions lateral with respect to said elongate aperture, saidprojections being positioned in a staggered array with respect to saidopposite directions to cause said cooling medium to flow in a zig zagfashion around said staggered projections.